1. Field of the Invention
The present invention relates to a system for controlling the gear changes of a vehicular automatic transmission and a method for executing the gear changes. More specifically, the present invention relates not only to a shift control system for an automatic transmission including series-connected first and second transmission assemblies, so that a downshift of the transmission may be achieved as a whole by simultaneously accomplishing a downshift for augmenting the gear ratio at the first transmission assembly and an upshift for reducing the gear ratio at the second transmission assembly, but also to a method for executing such shifts.
2. Discussion of the Background
In accordance with the rapid spread of the vehicular automatic transmission in recent years, there has been widely adopted, with a view to improving fuel consumption, a so-called "overdrive mechanism" having a gear ratio less than "1" which is connected as a second transmission assembly in series to a first transmission assembly capable of switching gear ratios automatically in relation to the running velocity and throttle opening of the vehicle.
In Japanese Patent Laid-Open No. 57-37140, there is disclosed an automatic transmission which is constructed to achieve six forward gear ratios by synchronizing the second transmission assembly or the overdrive mechanism positively with the shift of the first transmission assembly which is capable of achieving three forward gear ratios to shift the first and second transmission assemblies simultaneously or alternately. This automatic transmission is constructed to realize multiple gear ratios easily by setting the first transmission assembly at one of first to third speeds for each of the high and low-gear ratios of the second transmission assembly. In addition, the automatic transmission can achieve a number of advantages such as improvements in the fuel consumption and running performance and reductions in the loads upon frictional engagement means.
In this automatic transmission, however, the gear changes are performed by augmenting the gear ratio at the first transmission assembly and reducing the gear ratio at the second transmission assembly in the case of a downshift from the third to second speeds or from the fifth to fourth speeds. In other words, the gear changes are achieved by shifting the first and second transmission assemblies in the opposite directions to each other. If the gear changes, i.e., the shifts at the individual transmission assemblies were individually controlled in the case of such a downshift, the shifts could not be timed to augment shifting shocks. Another disadvantage would be an extraordinary drive feel which might otherwise be invited by a phenomenon that an upshift is caused at the second transmission assembly after a downshift at the first transmission assembly or vice versa.
On the other hand, we have revealed in Japanese Patent Laid-Open No. 62-165050 that excellent shifting characteristics can be obtained by starting and completing the substantial shift of the second transmission assembly during the substantial shift of the first transmission assembly. The substantial shift, as herein termed, is the period for which rotary members have their rotational frequency changed due to the shift, and is generally called the "inertia phase". Specifically, we have revealed in our Laid-Open specification that the start of the shift at the second transmission assembly should not occur before the start of the inertia phase of the first transmission assembly and that the end of the shift at the second transmission assembly should not occur after the end of the inertia phase of the first transmission assembly.
Let the case of a shift now be examined, in which the automatic transmission is to be shifted down in its entirety by performing a downshift for augmenting the gear ratio at the first transmission assembly and an upshift for reducing the gear ratio at the second transmission assembly. In this case, a timer is used to start the inertia phase of the second transmission assembly after the start of the inertia phase of the first transmission assembly. Generally speaking, the downshift has a smaller time lag from the output of a shift command to the actual start of the shift (i.e., the inertia phase) because it is achieved by releasing the frictional engagement means, whereas the upshift has a larger time lag from the output of the shift command to the start of the inertia phase because it is achieved by engaging the frictional engagement means. Thus, the timer is used for reflecting those time differences upon the shift controls. In case such a timer is used, the shift control system is constructed to output the upshift command having the larger time lag and then outputs the downshift command after a constant time has been counted by the timer. As a result, the inertia phase of the upshift is started immediately after the inertia phase of the downshift has been actually started.
In order to engage or release the aforementioned frictional engagement means, an oil pressure is generally used but will be inevitably disordered to cause failures of the timings for the gear changes. In case the oil pressure grows excessively high, for example, the frictional engagement means is prematurely engaged to advance the timing for the upshift to reduce the gear ratio and is released late, to retard the timing for the downshift to augment the gear ratio. On the other hand, a contrary phenomenon is invited in the case where the oil pressure drops. Thus, the timings for the upshift and downshift are highly displaced relative to each other no matter whether the oil pressure might be disordered to a high or low level.
Since, on the other hand, the engine torque to be inputted to the automatic transmission will inevitably fluctuate even for a constant throttle opening, the gear changing timings may be displaced. If the intake air temperature drops, for example, the engine has an increased output torque even for the constant throttle opening. In this case, a delay is liable to occur in the upshift to be achieved by engaging the frictional engagement means.
Moreover, the discrepancy in the lengths of the piston strokes of a hydraulic serve acting as an actuator for engaging or releasing the frictional engagement means will exert serious influences upon the discrepancy of the engaging timing of the frictional engagement means.
Even if, therefore, the shifting timings of the first and second transmission assemblies are to be controlled by the timer, their discrepancy is influenced in various manners by the aforementioned oil pressure or engine torque so that the optimizations of the two timings are restricted as a matter of course. In case the shifting timings are controlled exclusively by the timer, the aforementioned extraordinary shift feel may possibly occur to the driver if the dispersion of the oil pressure is serious.